function Get-AllowedEncoding {
    # targeting built-in Cmdlet
    $command = Get-Command -Name Microsoft.PowerShell.Utility\Out-File

    if ($PSVersionTable.PSVersion.Major -ge 6) {
        (
            $command.Parameters['Encoding'].Attributes |
                Where-Object { $_ -is [ArgumentCompletions] }
        )[0].CompleteArgument('Out-File', 'Encoding', '*', $null, @{ }).CompletionText
    }
    else {
        (
            $command.Parameters['Encoding'].Attributes |
                Where-Object { $_.TypeId -eq [ValidateSet] }
        )[0].ValidValues
    }
}

$EncodingCompleter = {
# missed param block
param(
	$commandName,
	$parameterName,
	$wordToComplete,
	$commandAst,
	$fakeBoundParameters
)

Get-AllowedEncoding |
	Where-Object { $_ -like "$wordToComplete*" } |
	ForEach-Object {
		[System.Management.Automation.CompletionResult]::new(
			$_,
			$_,
			[System.Management.Automation.CompletionResultType]::ParameterValue,
			$_
		)
	}
}

# Using the same completer for both functions
Register-ArgumentCompleter -CommandName Export-Ini -ParameterName Encoding -ScriptBlock $EncodingCompleter
Register-ArgumentCompleter -CommandName Import-Ini -ParameterName Encoding -ScriptBlock $EncodingCompleter